COPO was also used to get a Camaro built with a non-standard (non-RPO) option.An example is in 1968 4:56 rear gear was a RPO G82 in 1969 you had to get 4:56 gears using COPO 9511

I find this last statement about '69 very interesting. I took my rearend apart to rebuild it and found 456 gears in it. They are GM gears stamped 6/69 and my car was built first week of July. My POP says my gears should be 331. The car is a L48. Could these be COPO gears or what happened?

If it was ordered that way, the POP should show it. Possibly someone swapped an axle. Check the axle code on the front of the passenger side tube. I know I swapped a couple of boneyard axles back in the day. Fast cheap way to get a posi, or to fix a blown rear.